This invention relates to optical scanner devices, and more particularly to a scanner having a mirror which moves to deflect light along a scanning path.
Resonant scanners are used in scanned light beam devices, such as retinal display devices to scan light beams. In a retinal display device the scanner scans the light beam onto the retina of an eye to produce a perceived image. In an exemplary configuration for a retinal display device one scanner is used to provide horizontal deflection of a light beam, while another scanner is used to provide vertical deflection of the light beam. Together the two scanners deflect the light beam at changing angles to define a raster or other scanning pattern. By modulating the light beam and implementing multiple colors, a color image is scanned in raster format.
Each scanner includes a mirror which receives the light beam. The respective mirrors are moved in a periodic pattern over a prescribed angle. Such movement causes deflection of the light beam. For scanning a raster pattern the periodic pattern is repeated along the horizontal and vertical axes at respective scanning rates. The prescribed angle is referred to as a deflection angle and can be defined in a variety of fashions. For example, the deflection angle may be defined as total optical scan angle (TOSA) or peak mechanical scan angle (PMSA). In the context of a retinal scanning display the scanning rate and deflection angles are defined to meet the limits of the human eye. Analogous to refreshing a pixel on a display screen, the eye's retinal receptors must receive light from the scanning light beam periodically to continually perceive an ongoing image. Accordingly, the light beam rescans the image, or a changing image, in a periodic manner. The minimum refresh rate is a function of the light adaptive ability of the eye, the image luminance, and the length of time the retinal receptors perceive luminance after light impinges. To achieve television quality imaging the refresh rate typically is selected to be at least 50 to 60 times per second (i.e., .gtoreq.50 Hz to 60 Hz). Further, to perceive continuous movement within an image the refresh rate typically is at least 30 Hz.
To define a raster pattern in which thousands, or millions, of bits of information (e.g., light pixels) are communicated onto a small area (i.e., eye retina), the position of the mirror is controlled or monitored to a high degree of accuracy. In a conventional mechanical resonant scanner, the mirror is driven by a magnetic circuit that includes a pair of permanent magnets and a pair of electromagnets. Shortcomings of such a drive mechanism include undesirable weight of the magnets.
FIG. 1 shows a conventional scanner 10 having a mirror 12 and a spring plate 14. The mirror 12 and spring plate 14 are the only moving parts. The scanner 10 also includes a base plate 16 having a pair of stator posts 18, 20. Stator coils 22, 24 are wound in opposite directions about the respective stator posts 18, 20. The coil windings are connected in series or in parallel to a drive circuit. On opposite ends of the base plate 16, permanent magnets 26, 28 are mounted equidistant from the posts 18, 20. The spring plate 14 has enlarged opposite ends 30 that rest on a pole of a respective permanent magnet. The magnets are oriented to have the same pole in contact with each end of the spring plate 14. Thus, the opposite pole of each magnet 26, 28 is located adjacent to the base plate 16. The spring plate 14, magnets 26, 28 and the base plate 16 are tightly clamped together by respective caps 34, 36.
Magnetic circuits are formed in the scanner 10 to oscillate the mirror 12 about an axis of rotation 15. A first magnetic circuit extends from the top pole of the magnet 26 to the spring plate end 30, through an arm of the spring plate and mirror 12 across a gap to the stator pole 18, then through the base plate 16 back to the permanent magnet 26. A second magnetic circuit extends a similar path but through the stator post 20 instead of the stator post 18. A third magnetic circuit extends from the top pole of the magnet 28 to the opposite spring plate end 30, through an arm of the spring plate and mirror 12 across a gap to the stator pole 18, then through the base plate 16 back to the permanent magnet 28. A fourth magnetic circuit extends a similar path but through the stator post 20 instead of the stator post 18. A periodic drive signal is applied to the oppositely wound coils 22, 24 creating magnetic fields which cause the mirror 12 to oscillate back and forth about the axis of rotation 15. The phase angle of the mirror is not detected. A pair of frequency adjustment screws 37, 38 can be adjusted to increase or decrease the tension in the spring plate 14. Variation of such tension increases or decreases the resonant frequency of the scanner 10.